If you add the exchange 2003 server to the existing domain, just leave the domain in win2000/exchange 2000 interop mode until you remove the exchange 2000 server. Follow the normal MS procedures to move a first exchange server in an org to another server.
Perform mailbox moves to recreate the mailboxes on the new server. Don’t forget to add the registry key to bump up the store size to 75gb or whatever you wish. Also be careful as if you move 30gb worth of mailboxes between servers, it will create around 30gb of log files on both servers. Make sure you have the log space and perform backups to truncate the logs or you could run out of disk space (not fun).
I would also recommend upgrading active directory to 2003 native mode and then flip to exchange server 2003 native mode as soon as is convenient for you. The upgrade of AD from win 2000 to 2003 is very easy.